Lost all brake fluid while truck was parked.

So the truck sat about a week or two, parked on a slight hill but I can't remember which way.  When I went to drive it I had almost no brakes, so I went back home and checked the reservoir first and there was no fluid left at all.  It was good when it was parked.  I've never come across something like this before, and I'm not seeing any obvious leaks at the wheels.  The rear cylinders are someone new, but the front calipers and all other brake parts are unknown age.  

One other question, this is the 8500#+ GVWR truck, which has a different booster, but those are impossible to find.  If that's the issue, would the standard booster work fine, maybe just less power?  I never tow with the truck or haul much of a load so the brakes are already more than I need.

why are you asking about the booster?  you must suspect something

I'm thinking about replacing everything all at once, and that's the only thing that I had a question about.  

Mine failed while I was driving the truck, but not at the wheel cylinders or calipers. For whatever reason, it failed where the hard line on the left rear wheel went into the splitter block on the rear axle housing.

I'm not saying that's where yours failed, I'm just saying the hydraulic system could fail just about anywhere. I'd put some more fluid in it, and park in on cement, then see where wet spots show up.

I'm assuming you didn't notice any wet spots after it lost it's fluid. Was it parked on grass or dirt, where it would have been hard to see?

Yeah, there's no cement anywhere near me.  All dirt, pine straw and gravel.  I'd like to replace all the lines while I'm at it, I'm not having much luck finding steel lines though.  

As said fill it back up and have a helper push the pedal while you look under the truck for leaks.

Where both sides of the master empty?
I dont think I have seen both sides empty from just sitting?
If no leaks then the fluid went into the booster and that is not a good thing.
Could pull the booster off and see if anything drains out of it.
Masters have been known to leak into the booster.
Dave ----

about twenty years ago i had a case where i was doing roofing regularly. business was good after a tornado in nashville. my half ton shortbed ran constantly loaded with tools and ladders while pulling loaded trailers of either new or waste shingles. i started going through brake fluid but never saw a leak. one day while at the disposal station. sitting on a lot of dust i saw one little drop. a coupling in the line between the frame rail and fuel tank was not leaking unless under heavy braking and i can only imagine it nearly atomizing it. anyway, it was finally enough to find. Brake fluid does dry even if it does slowly. look very close at every joint and pay more attention to rust patterns